feat(layout,test): resolve backlog items I-2, I-3, and C-1 with unanimous peer review

- I-2: add 5.0s upper-bound execution time assertion in test_bug4_headless_unobservable_fast_path to contractually guard SKS_EMPTY_GIVEUP early-exit latency
- I-3: clean up PaneInfo.focused, wire MAM_MAX_PANE_COLS/MAM_MAX_COLS environment variables and CLI --max-cols flag
- C-1: apply max_columns growth guard to headless 0x0 layouts, mirroring GUI behavior
- Promoted Planner consensus plan Rev.2 (plan-fea5f1b2.md) and unanimous PASS reports from Reviewers Claude (report-55d1a1d9.md) and Cline (report-6f18ba0f.md)
- Verified all 333 test cases pass with exit code 0

tests/test_b19_headless_reconcile_fixes.py

@@ -2,6 +2,7 @@ import os
 import sys
 import json
 import subprocess
+import time
 import pytest
 
 from lib_py.layout import compute_2xk_layout
@@ -129,7 +130,14 @@ echo "SUCCESS"
 
 
 def test_bug4_headless_unobservable_fast_path(tmp_path):
-    """Verify Bug 4 / R-1: in headless mode where capture-pane is empty, send_keys_safe bypasses dialogs and succeeds immediately via RPC fast-path."""
+    """Verify Bug 4 / R-1 + I-2: in headless mode where capture-pane is empty,
+    send_keys_safe bypasses dialogs and succeeds immediately via the RPC fast-path.
+
+    The elapsed-time bound is a contract, not a nicety: removing the
+    SKS_EMPTY_GIVEUP early exit leaves every functional assertion green and only
+    changes the wall clock (measured 1.22s -> 10.21s), so this is the sole
+    assertion that can detect that regression.
+    """
     test_script = f"""#!/usr/bin/env bash
 set -euo pipefail
 SKILL_DIR="{os.path.abspath('.agents/skills')}"
@@ -149,7 +157,6 @@ _sks_herdr() {{
   fi
   if [ "${{1:-}}" = "paste-buffer" ]; then
     PASTE_CALLED=1
-    return 0
   fi
   return 0
 }}
@@ -166,9 +173,21 @@ if [ "$PASTE_CALLED" = "1" ]; then
 fi
 echo "HEADLESS_OK"
 """
-    res = subprocess.run(["bash", "-c", test_script], capture_output=True, text=True)
+    # Remove SKS_* from env so lib.sh defaults apply cleanly
+    env = {k: v for k, v in os.environ.items()
+           if k not in ("SKS_QUIESCENT_TRIES", "SKS_QUIESCENT_INTERVAL", "SKS_EMPTY_GIVEUP")}
+
+    t0 = time.perf_counter()
+    res = subprocess.run(["bash", "-c", test_script], capture_output=True, text=True, env=env)
+    elapsed = time.perf_counter() - t0
+
     assert res.returncode == 0, f"Headless send_keys_safe failed: {res.stderr}"
     assert "HEADLESS_OK" in res.stdout
+    assert elapsed < 5.0, (
+        f"headless fast-path took {elapsed:.2f}s (limit 5.0s) — the "
+        f"SKS_EMPTY_GIVEUP early exit in _pane_quiescent is likely gone; "
+        f"the full 10s quiescence window was consumed instead"
+    )
 
 
 def test_bug4_slow_settling_pane_success(tmp_path):